How to upscale final render (compositing)
I don't know a way to do this without saving out your render as an image first. Basically, you'll render your image at 480 x 270 as normal, and save as an image. Then bring the render into the compositor; use the transform node with the "Nearest" setting from the dropdown menu; and for scale, just scale it enough to match the new resolution (480 x 270 to 1920 x 1080 is a scale value of 4).